To reduce costs to a minimum level, making solar systems a viable domestic option, the materials, dimensions, and method of fabrication must be chosen with great care. Therefore, alternative methods of construction need to be looked into.

A typical flat plate collector is made from a number of parts. To simplify the construction and reduce costs an extruded polycarbonate structure is to be incorporated into the design of the flat plate collector. The purpose of the extruded triple walled polycarbonate construction is to substitute for both the glass cover and the absorber plate. Figure 3.1.1, shows a section of the extruded polycarbonate panel.

Both types of thermal collector configurations being looked at in this study are shown as a section in figure

3.1.2 and 3.1.3. Figure 3.1.2 shows the set up of the collector with a black — coated backing between the insulation and the polycarbonate. Figure 3.1.3, shows the collector with an aluminum backing (absorber) between the insulation and the polycarbonate.

Under steady state conditions the energy balance is used to describe the performance of a solar collector. The useful energy output of a collector of area Ac is the difference between the absorbed solar radiation and the thermal loss [1]:

Qu = A [5 — UL (Tpm — Ta)]

The overall heat loss from a solar collector, UL, consists of top heat loss through cover systems and back and edge heat loss through back and edge insulation of the collector can be expressed as:

Ul = Utop + UEdge + UBack (3’1’2

Since polycarbonate material is being used instead of glass, it is necessary to calculate UTop for a plastic glazing, in order to analyse the thermal performance of the solar collector. The net radiation method was used to calculate the top loss coefficient of plastic-covered solar flat-plate collectors. To evaluate the heat loss through the cover system, all of the convection and radiation heat transfer mechanisms between parallel plates and between the plate and the sky must be considered as shown in Figure 3.1.

Analysing each of the variable

Performing an energy balance on the cover, Utop can be easily obtained:

Q£ox? ftp ^"Qcw, to Qcovrfip Qc/Ntrfxt ^Qsoieypr ‘oer fib/т/

QLve’,dsflnf Ьр-с (p T)

Or* =Ш ~7I)

Qpjcss Q, dsthr ^Qovrjisficnr Qphafar

U Qfils-/ tp~ T _TJ

(3.1.11)

When evaluating the back heat loss, the thermal resistances from convection and radiation heat transfer are much smaller than that of conduction. Therefore, it can be assumed that all the thermal resistance from the back is due to the insulation. The back heat loss, Qb, can be obtained from:

Qb — kb Ac (Tpm-Ta)

Lb (3.1.12)

With the assumption of one-dimensional sideways heat flow around the perimeter of the collector, the edge losses can be estimated by

Qe — kg Ae (Tpm-Ta)

Le (3.1.13)